The present invention relates to a method and apparatus for making a spark plug.
In a spark plug for use in an internal combustion engine, the accuracy in positioning a ground electrode and a center electrode is important. For example, there possibly occurs such a case in which the center lines of the ground electrode and center electrode with a spark gap being disposed therebetween are eccentric due to defective bending of the ground electrode, displacement of a noble metal attached to one of or each of the electrodes, etc. Such eccentricity will lead to, for example, a short life due to a partial electrode consumption and a trouble of misfire. Further, a spark gap larger than a predetermined value requires an excessively high discharge voltage so as to disable the spark plug to fire. On the other hand, a spark gap smaller than a predetermined value tends to cause short of the spark gap, etc.
For adjustment of the eccentricity between the electrodes and the spark gap, it is known a method of treating the ground electrode by an adjustment bending by means of a bending machine having a pressing punch. For example, in Japanese Patent Unexamined Publication No. 3-64882 is disclosed a method of repeatedly pressing the ground electrode by means of a pressing punch until the spark gap is adjusted to a target value while monitoring the spark gap by means of a CCD camera. In this instance, a target spark gap is set smaller than an ideal spark gap by a predetermined value with consideration of spring back of the ground electrode upon release of a pressing force on the ground electrode.
In Japanese Patent Unexamined Publication No. 11-121143 is disclosed a method of calculating the eccentricity of the center line of the noble metal chip disposed at the ground electrode and the center line of the center electrode after inspection of the spark gap and adjusting the position of the ground electrode in the width direction thereof.